<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>居中显示</title>
    <style>
        .outer{
   width: 500px;
   height: 500px;
   background-color: aqua;
   position: relative;
        }
        .inner{
            width: 200px;
   height: 200px;
   background-color:bisque;
   margin: auto;
   top:0;
   bottom:0;
   left:0;
   right:0;
   position: absolute;
        }
    </style>
</head>
<body>
    <div class="outer">
        <div class="inner"></div>
    </div>
</body>
</html>